III-N semiconductors are promising optoelectronic materials whose direct bandgap optical emission spans the ultraviolet to infrared. Despite the impressive progress of visible light-emitting diodes (LEDs) in the nitride material system over the past two decades, two major issues are widely recognized: 1) the green gap, and 2) efficiency droop. The green gap refers to the fact that today's InGaN LEDs emitting in the green have lower efficiencies than comparable devices in the blue, or to red LEDs in the AlInGaP material system. This is an issue since the sensitivity of the human eye peaks in the green. Efficiency droop is the reduction in efficiency at high drive levels, suitable for illumination. The exact origins of these effects are not fully understood, but often the polarization fields in the InGaN wurtzite material system are thought to be a root cause.
GaN and alloys including AlGaN and InGaN (or GaInN) exhibit both hexagonal (wurtzite) and cubic (e.g., zinc-blende) phases. As used herein, the terms AlGaN, InGaN and GaInN are shorthand notations common in the field for AlxGa1-xN, InyGa1-yN and Ga1-yInyN. For GaN and InGaN the hexagonal (h-GaN) phase is energetically preferred and with a few exceptions, all device applications including high-power transistors, light-emitting diodes (LEDs) and laser diodes have been developed with h-GaN material.
There are potential advantages to cubic (c-GaN) material. One advantage is that the (001) direction (i.e., the direction of growth on the (001) crystal face) is not only free of spontaneous polarization, but also free of piezo-electric polarization. The bandgap of the cubic nitrides is slightly smaller than that of the wurtzite polytypes, and therefore the band-edge emission in bulk crystals of cubic GaInN has longer wavelength than in wurtzite crystals with the same indium concentration. The general absence of internal electric fields in c-GaN LEDs can eliminate the redshift associated with the quantum-confined Stark effect (QCSE).
Another potential benefit of cubic GaN is the high hole mobility, which is reported to reach 350 cm2V−1s−1 in cubic GaN on GaAs. A practical advantage of (001) c-GaN is that it can be cleaved along the {110} planes, which are perpendicular to the (100) growth direction. This could be a major advantage for device fabrication, e.g. the fabrication of laser diode devices. Further, based on theoretical calculations, it has been suggested that the Auger-recombination in the blue-green region could be smaller in c-GaInN structures than in their wurtzite counter-part. This could impact the efficiency droop effects.
According to the literature, c-GaN has been successfully grown by plasma assisted molecular beam epitaxy (MBE) or MOVPE on different substrates, including 3C—SiC, 6H—SiC (as super lattice), GaAs, and Si (001). Also, growth of nano-wires with cubic GaN has been demonstrated by MBE. In the case of growth on GaAs, large free-standing samples with 100 μm thickness have been achieved. Novikov et al., “Molecular beam epitaxy as a method for the growth of freestanding zinc-blende (cubic) GaN layers and substrates,” J. Vac. Sci. Technol. B 28, vol. 28, no. 3, pp. C3B1-C3B6, October 2010. However, it took those authors around 8 days to grow such material. It remains difficult to keep optimal growth conditions for such long durations, and the best material quality was believed to have been achieved only within the first 10 μm starting from the substrate. This thickness however would be sufficient for use in LEDs.
Novikov also recently demonstrated the feasibility of growing freestanding cubic GaN with a thickness of more than 50 μm on a GaAs substrate of 7.62 cm (3 inch) diameter. “Zinc-blend (cubic) GaN bulk crystals grown by molecular beam epitaxy,” Phys. Stat. Sol. (c), vol. 8, no. 5, pp. 1439-1444, May 2011. Novikov also demonstrated cubic AlxGa1-xN layers in a very wide range of alloy compositions. While these are impressive results, it is generally agreed that such long growths are not compatible with cost-effective manufacturing of LEDs in high volume.
Structures with cubic GaInN/GaN multi-quantum wells (MQW) have been reported on 3C SiC, where PL emission was observed up to a wavelength of 520 nm. Simple p-GaN/n-GaN junction LEDs have been reported on GaAs. The electroluminescence (EL) emission at 430 nm was attributed to originate from impurity-related recombination and its intensity showed a linear dependence on the drive current density in the range of 50-300 A/cm2. A p-GaN/i-GaN/n-AlGaN junction LED (emission at 477 nm) as well as a double heterojunction LED with GaInN active layer grown on GaAs (emission at 430 nm and 470 nm) have been demonstrated. It can be inferred from the published EL spectra of these devices that the intensity depends linearly on the drive current. However, the total light output power from these devices is not known.
Recently, a GaInN/GaN LED grown by ammonia-MBE has been demonstrated. It was grown on freestanding cubic GaN templates by MBE on GaAs and showed EL around 460 nm. There is also published work on short wavelength devices. Near UV emission at 370 nm in PL from cubic AlGaN/GaN multi-quantum wells with varying width can be modeled using square-well potentials, which was interpreted as absence of polarization fields along the (001) direction.
It is fair to say that the above described techniques for growing c-group-III-N compounds have involved isolated efforts that have not been suitable and accepted for device fabrication. Many of the results showed significant levels of defects including uncontrolled spatial variations between c-GaN and h-GaN materials. The h-GaN material remains the better material, the most explored for device applications, and the only phase used for today's commercial devices.
The epitaxial growth of high quality, c-group-III-N compounds at a physical scale applicable to practical device fabrication is also not well established. This is at least partially due to various problems such as uncontrolled phase mixtures with the hexagonal phase and some issues on the selection of substrates for epitaxy related to the mismatch in crystal symmetry and lattice constant. Generally, sapphire and SiC have been employed as substrate materials but these are incompatible with the mainstream semiconductor technology that exclusively uses Si(001) substrates. In spite of its predominant use for microelectronics, Si has not been extensively investigated as a substrate for c-group-III-Nitrides because of these growth problems, and, for optical applications, because of the intrinsic absorption in any remaining Si after the growth. A successful use of Si as a substrate for c- or h-III-N optical devices would desirably include a process for removal of the substrate material before the device packaging.
An emerging theme in modern crystal growth is the integration of the exquisitely controlled growth capabilities of MBE and MOCVD with developing large-area nanoscale lithography capabilities. The useful length scale for the lithography is less than or comparable to an adatom diffusion length during growth. Recently, relying on large-area nanoscale interferometric lithography, the inventors of this disclosure demonstrated the growth of c-GaN with a controllable, symmetry-induced phase separation from the hexagonal phase during growth on a deep sub-micron scale Si(111)-faceted v-groove fabricated into a Si(001) substrate. S. C. Lee, et al., Appl. Phys. Lett. 84 (2004) 2079. This proves the availability of the epitaxial growth of c-group-III-N materials on a Si(001) substrate at the nanoscale regime and is directly compatible with current Si microelectronics technology.
Further advancements in growth of c-group-III-N materials that overcome one or more of the deficiencies of current growth techniques, such as those mentioned above, would be a desirable addition to the field of III-N semiconductors.